The present invention relates to a semiconductor device such as a short-wavelength light emitting diode device or a short-wavelength semiconductor laser device, and a method for manufacturing the same.
A semiconductor material made of a group III-V nitride semiconductor, having a wide forbidden band, can be used in light emitting devices, specifically, light emitting diode devices and short-wavelength semiconductor laser devices that are capable of emitting light of a color in a visible region such as blue, green or white. Among others, light emitting diode devices have already been in practical use in large-size display apparatuses, traffic signals, etc. Particularly, white light emitting diode devices, which give white light by exciting a fluorescent substance, are expected to replace conventional lighting fixtures such as electric bulbs and fluorescent lamps. Moreover, the development of semiconductor laser devices has reached a point where samples are being shipped and products are being manufactured although in small quantities, for use in high-density, large-capacity optical disk apparatuses using blue-violet laser light.
The crystal growth of a group III-V nitride semiconductor, or a so-called xe2x80x9cgallium nitride (GaN) semiconductorxe2x80x9d, has been difficult, as is also the case with other wide gap semiconductors. However, with the recent significant improvements in crystal growth techniques such as a metal organic chemical vapor deposition method, light emitting diode devices capable of emitting light of short wavelengths such as blue light have already been in practical use.
Moreover, since a substrate made of gallium nitride is difficult to produce, a gallium nitride semiconductor cannot be grown by a crystal growth technique that is used with silicon (Si) or gallium arsenide (GaAs), i.e., growing a semiconductor layer (epitaxial growth layer) on a substrate having the same composition as that of the semiconductor layer. Therefore, a so-called xe2x80x9cheteroepitaxial growth processxe2x80x9d is typically employed, in which the epitaxial growth layer is grown on a substrate having a different composition from that of the epitaxial growth layer, e.g., a sapphire substrate.
As a result, a gallium nitride semiconductor layer grown on a sapphire substrate is currently exhibiting the most desirable device characteristics, where the crystal defect density of the epitaxial growth layer is about 1xc3x97107 cmxe2x88x922. However, since sapphire is insulative, in order to form a device including a p-n junction on a substrate made of sapphire, it is necessary to selectively remove the p-type semiconductor layer or the n-type semiconductor layer after the epitaxial growth and to form a p-type electrode and an n-type electrode on the principal surface of the substrate.
Moreover, since it is typically difficult to perform a wet etching process with an acidic solution, or the like, on a nitride semiconductor, a dry etching method such as reactive ion etching is normally used in such a selective removal step.
A method for manufacturing a semiconductor device according to a first conventional example will now be described with reference to the drawings.
FIG. 21 is a cross-sectional view illustrating a light emitting diode device, which is a semiconductor device of the first conventional example.
As illustrated in FIG. 21, first, a buffer layer (not shown) made of gallium nitride or aluminum nitride, an n-type cladding layer 102 made of n-type aluminum gallium nitride, an active layer 103 including a quantum well structure made of undoped indium gallium nitride, and a p-type cladding layer 104 made of p-type aluminum gallium nitride are grown in this order on a substrate 101 made of sapphire by a metal organic chemical vapor deposition method, or the like, to form an epitaxial layer. As a current is externally injected into the n-type cladding layer 102 and the p-type cladding layer 104, electrons and holes are confined in the active layer 103, and output light is produced through recombination of electrons and holes.
Then, the p-type cladding layer 104, the active layer 103 and an upper portion of the n-type cladding layer 102 are selectively etched by a reactive ion etching method to form a current constriction section 200 in the epitaxial layer. Then, the p-side electrode 105 is formed on the p-type cladding layer 104 in the current constriction section 200, and an n-side electrode 106 is formed on the exposed region of the n-type cladding layer 102.
FIG. 22 is a cross-sectional view illustrating a semiconductor laser device, which is a semiconductor device of the second conventional example.
As illustrated in FIG. 22, in order to produce a semiconductor laser device, an upper portion of the current constriction section 200 is again subjected to a reactive ion etching method to form a ridge portion 201 to be a waveguide, and then the p-side electrode 105 is formed in a stripe pattern. Furthermore, the structure is cleaved along a plane perpendicular to the direction in which the p-side electrode 105 having a stripe pattern extends, thereby forming a cavity with the two opposing cleaved surfaces being mirrors. Herein, the upper surface excluding the p-side electrode 105 and the n-side electrode 106 is covered by an insulating film 107 made of silicon oxide.
However, with the methods for manufacturing a semiconductor device of the first and second conventional examples, a nitride semiconductor layer for forming the current constriction section 200 needs to be subjected to a dry etching process. The dry etching process damages the side surfaces of the current constriction section 200. With such a damage, when a current is supplied through the semiconductor device, a leakage current occurs through the damaged portions, thereby increasing the operating current of a light emitting diode device, or the threshold current value of a semiconductor laser device.
Moreover, as described above, sapphire, which is insulative, is used for the substrate 101, whereby both of the p-side electrode 105 and the n-side electrode 106 need to be formed on the principal surface of the substrate 101. This increases the series resistance value as a p-n junction, while increasing the device cost because of an increase in the chip area.
Moreover, sapphire has a relatively small thermal conductivity, and thus a poor heat radiating property. Therefore, when a semiconductor laser device, for example, is produced using sapphire, it is difficult to increase the operating lifetime of the semiconductor laser device.
In view of these problems in the prior art, a first object of the present invention is to provide a semiconductor device using a group III-V nitride semiconductor, in which a current constriction section can be formed without damaging an exposed surface (side surface) of an active region. Moreover, a second object of the present invention is to reduce the series resistance value and improve the heat radiating property.
In order to achieve the first object, the present invention employs a structure in which a semiconductor layer including an active region is oxidized at positions spaced apart from each other to form oxidized regions so that the oxidized regions form a current constriction section. Moreover, even when the semiconductor layer is dry-etched, the side surface of the current constriction section is oxidized.
Moreover, in order to achieve the second object in addition to the first object, a semiconductor layer is formed on a substrate so that an active region is included in the semiconductor layer, after which the substrate is removed from the semiconductor layer.
Specifically, a semiconductor device of the present invention, which achieves the first object, includes a first semiconductor layer of a first conductivity type and a second semiconductor layer of a second conductivity type, including an active region, wherein at least one of the first semiconductor layer and the second semiconductor layer includes oxidized regions, which are spaced apart from each other in a direction parallel to a plane of the active region and are obtained through oxidization of the at least one of the first semiconductor layer and the second semiconductor layer itself.
With the semiconductor device of the present invention, the oxidized regions are formed so as to be spaced apart from each other in the direction parallel to the plane of the active region, whereby the oxidized regions function as a current constriction structure. Furthermore, the oxidized regions are obtained through oxidization of the first semiconductor layer or the second semiconductor layer itself, whereby it is not necessary to use a dry etching process for the current constriction structure, thus preventing the current constriction structure from being damaged. As a result, it is possible to prevent a leakage current occurring in the active region via a damaged portion.
It is preferred that the semiconductor device of the present invention further includes: a first ohmic electrode formed on the second semiconductor layer; and a second ohmic electrode formed on one side of the first semiconductor layer that is away from the second semiconductor layer. In this way, the second object is achieved.
In the semiconductor device of the present invention, it is preferred that a conductive substrate is provided between the first semiconductor layer and the second ohmic electrode.
In such a case, it is preferred that the conductive substrate is made of silicon carbide, silicon, gallium arsenide, gallium phosphide, indium phosphide, zinc oxide or a metal.
In the semiconductor device of the present invention, it is preferred that the first semiconductor layer and the second semiconductor layer are formed in this order on an insulative substrate, the semiconductor device further including: a first ohmic electrode formed on the second semiconductor layer; and a second ohmic electrode formed on an exposed portion of one surface of the first semiconductor layer that is closer to the second semiconductor layer.
In such a case, it is preferred that the insulative substrate is made of sapphire, magnesium oxide or lithium gallium aluminum oxide (LiGaxAl1xe2x88x92xO2 (where 0xe2x89xa6xxe2x89xa61)).
In the semiconductor device of the present invention, it is preferred that the oxidized regions are formed so as to include the active region.
In the semiconductor device of the present invention, it is preferred that at least one of the first semiconductor layer and the second semiconductor layer includes a current constriction section formed by removing side portions of the at least one of the first semiconductor layer and the second semiconductor layer.
In such a case, it is preferred that a ridge portion to be a waveguide is formed in an upper portion of the current constriction section.
In the semiconductor device of the present invention, it is preferred that an insulating film is formed on the oxidized regions.
In such a case, it is preferred that the insulating film is made of silicon oxide or silicon nitride.
In the semiconductor device of the present invention, it is preferred that the first semiconductor layer and the second semiconductor layer are made of a compound semiconductor containing nitrogen.
A first method for manufacturing a semiconductor device of the present invention, which achieves the first object, includes: a first step of forming a first semiconductor layer of a first conductivity type; a second step of forming a second semiconductor layer of a second conductivity type on the first semiconductor layer, thereby forming an active region between the first semiconductor layer and the second semiconductor layer; and a third step of selectively oxidizing at least the second semiconductor layer, thereby forming, at least in the second semiconductor layer, oxidized regions spaced apart from each other in a direction parallel to a plane of the active region.
With the first method for manufacturing a semiconductor device, the oxidized regions are formed so as to be spaced apart from each other in the direction parallel to the plane of the active region, whereby the oxidized regions function as a current constriction section. In addition, the oxidized regions are obtained through oxidization of the second semiconductor layer itself, whereby it is not necessary to use a dry etching process for forming the current constriction section, thus preventing an etching damage to the current constriction section. As a result, it is possible to prevent a leakage current occurring in the active region via a damaged portion.
In the first method for manufacturing a semiconductor device, it is preferred that the third step includes a step of selectively covering an upper surface of the second semiconductor layer by a mask film made of a material that is less likely to be oxidized than the second semiconductor layer.
In such a case, it is preferred that the first method for manufacturing a semiconductor device further includes, after the third step, a fourth step of forming an ohmic electrode on the second semiconductor layer after removing the mask film.
It is preferred that the first method for manufacturing a semiconductor device further includes, after the third step: a fourth step of forming a first ohmic electrode on the second semiconductor layer; and a fifth step of forming a second ohmic electrode on one surface of the first semiconductor layer that is away from the active region. In this way, the second object is achieved.
It is preferred that the first method for manufacturing a semiconductor device further includes, after the third step: a fourth step of forming a first ohmic electrode on the second semiconductor layer; and a fifth step of selectively removing the active region and the second semiconductor layer, thereby forming an exposed region of the first semiconductor layer, and forming a second ohmic electrode on the formed exposed region.
In such a case, it is preferred that the fourth step includes: a step of forming an insulating film on the second semiconductor layer including the oxidized regions; a step of forming a resist pattern having an opening corresponding to a portion of the insulating film above the second semiconductor layer, and then etching the insulating film while using the formed resist pattern as a mask, thereby transferring an opening pattern onto the insulating film; and a step of depositing a metal film on the second semiconductor layer including the resist pattern, and lifting off the resist pattern, thereby forming the first ohmic electrode from the metal film.
In the first method for manufacturing a semiconductor device, it is preferred that the insulating film is made of silicon oxide or silicon nitride.
In the first method for manufacturing a semiconductor device, it is preferred that in the first step, the first semiconductor layer is formed on a substrate; and the method further includes, after the third step, a step of separating the substrate from the first semiconductor layer. In this way, the second object is achieved.
In such a case, it is preferred that the first method for manufacturing a semiconductor device further includes, between the second step and the third step, a fourth step of etching at least the second semiconductor layer, thereby forming a current constriction section having a convex cross section at least in the second semiconductor layer.
Moreover, in such a case, it is preferred that in the fourth step, the current constriction section is formed so as to reach the first semiconductor layer.
Alternatively, in such a case, it is preferred that in the fourth step, the current constriction section is formed so as not to reach the active region.
Moreover, in such a case, it is preferred that the fourth step includes a step of forming a ridge portion to be a waveguide in an upper portion of the second semiconductor layer within the current constriction section.
In the first method for manufacturing a semiconductor device, it is preferred that in the third step, the oxidization is performed in an atmosphere containing an oxygen gas or water vapor.
A second method for manufacturing a semiconductor device of the present invention includes: a first step of forming a portion of a first semiconductor layer of a first conductivity type; a second step of selectively oxidizing the portion of the first semiconductor layer, thereby forming, in the portion of the first semiconductor layer, oxidized regions spaced apart from each other in a direction parallel to a plane of the first semiconductor layer; a third step of forming a rest of the first semiconductor layer on the portion of the first semiconductor layer including the oxidized regions; and a fourth step of forming a second semiconductor layer of a second conductivity type on the first semiconductor layer, thereby forming an active region between the first semiconductor layer and the second semiconductor layer.
With the second method for manufacturing a semiconductor device, the oxidized regions to be the current constriction section are formed in a portion of the first semiconductor layer, and then the rest of the first semiconductor layer, the active region and the second semiconductor layer are formed. Therefore, as with the first method for manufacturing a semiconductor device, it is not necessary to use a dry etching process for forming the current constriction section, thus preventing an etching damage to the current constriction section. As a result, it is possible to prevent a leakage current occurring in the active region via a damaged portion.
In the second method for manufacturing a semiconductor device, it is preferred that the second step includes a step of selectively covering an upper surface of the portion of the first semiconductor layer by a mask film made of a material that is less likely to be oxidized than the first semiconductor layer.
In such a case, it is preferred that the second method for manufacturing a semiconductor device further includes: a fifth step of removing the mask film, between the second step and the third step; and a sixth step of forming an ohmic electrode on the second semiconductor layer, after the fourth step.
It is preferred that the second method for manufacturing a semiconductor device further includes, after the fourth step: a fifth step of forming a first ohmic electrode on the second semiconductor layer; and a sixth step of forming a second ohmic electrode on one surface of the first semiconductor layer that is away from the active region.
In the second method for manufacturing a semiconductor device, it is preferred that in the first step, the portion of the first semiconductor layer is formed on a substrate; and the method further includes, after the fourth step, a step of separating the substrate from the first semiconductor layer. In this way, the second object is achieved.
In the second method for manufacturing a semiconductor device, it is preferred that in the second step, the oxidization is performed in an atmosphere containing an oxygen gas or water vapor.
A third method for manufacturing a semiconductor device of the present invention includes: a first step of forming a first semiconductor layer of a first conductivity type; a second step of forming a portion of a second semiconductor layer of a second conductivity type on the first semiconductor layer, thereby forming an active region between the first semiconductor layer and the second semiconductor layer; a third step of selectively oxidizing the first semiconductor layer, the active region and the portion of the second semiconductor layer, thereby forming oxidized regions spaced apart from each other in a direction parallel to a plane of the second semiconductor layer, in the first semiconductor layer, the active region and the portion of the second semiconductor layer; and a fourth step of forming a rest of the second semiconductor layer on the portion of the second semiconductor layer including the oxidized regions.
With the third method for manufacturing a semiconductor device, the oxidized regions to be the current constriction section are formed in the first semiconductor layer, the active region and a portion of the second semiconductor layer, and then the rest of the second semiconductor layer is formed. Therefore, as with the second method for manufacturing a semiconductor device, it is not necessary to use a dry etching process for forming the current constriction section, thus preventing an etching damage to the current constriction section. As a result, it is possible to prevent a leakage current occurring in the active region via a damaged portion.
In the third method for manufacturing a semiconductor device, it is preferred that in the third step, the oxidization is performed in an atmosphere containing an oxygen gas or water vapor.
In the first or second method for manufacturing a semiconductor device, it is preferred that the substrate is made of sapphire, silicon carbide, silicon, gallium arsenide, gallium phosphide, indium phosphide, magnesium oxide, zinc oxide or lithium gallium aluminum oxide (LiGaxAl1xe2x88x92xO2 (where 0xe2x89xa6xxe2x89xa61)).
In the first or second method for manufacturing a semiconductor device, it is preferred that the substrate separation step includes a step of bonding a support substrate for supporting the second semiconductor layer to an upper surface of the second semiconductor layer.
In such a case, it is preferred that the first or second method for manufacturing a semiconductor device further includes, after the substrate separation step, a step of forming an ohmic electrode on the support substrate.
In such a case, it is preferred that the support substrate is made of silicon, gallium arsenide, gallium phosphide, indium phosphide or a metal.
In the first or second method for manufacturing a semiconductor device, it is preferred that the substrate separation step is performed by a polishing method.
In the first or second method for manufacturing a semiconductor device, it is preferred that the substrate is made of a material whose forbidden band width is larger than that of the first semiconductor layer; the substrate separation step includes a step of irradiating the first semiconductor layer with irradiation light from one surface of the substrate that is away from the first semiconductor layer; and an energy of the irradiation light is smaller than the forbidden band width of the substrate and larger than that of the first semiconductor layer.
Moreover, in the first or second method for manufacturing a semiconductor device, it is preferred that the first semiconductor layer is made of a plurality of semiconductor layers having different compositions; the substrate is made of a material whose forbidden band width is larger than a forbidden band width of one of the plurality of semiconductor layers that has a smallest forbidden band width; the substrate separation step includes a step of irradiating the first semiconductor layer with irradiation light from one surface of the substrate that is away from the first semiconductor layer; and an energy of the irradiation light is smaller than the forbidden band width of the substrate and larger than the forbidden band width of one of the plurality of semiconductor layers that has the smallest forbidden band width.
In such cases, it is preferred that the irradiation light is laser light that oscillates in a pulsed manner.
Alternatively, it is preferred that the irradiation light is an emission line of a mercury lamp.
Moreover, in such a case, it is preferred that the substrate separation step includes a step of heating the substrate.
In the first or second method for manufacturing a semiconductor device, it is preferred that in the substrate separation step, the irradiation light is radiated so as to scan a surface of the substrate.
In the first to third methods for manufacturing a semiconductor device, it is preferred that the first semiconductor layer and the second semiconductor layer are deposited by using one of a metal organic chemical vapor deposition method, a molecular beam epitaxy method and a hydride vapor phase epitaxy method, or by using more than one of the methods in combination.
In the first to third methods for manufacturing a semiconductor device, it is preferred that the first semiconductor layer and the second semiconductor layer are made of a compound semiconductor containing nitrogen.